Metallic grid-stiffened structures have been used in aerospace and other industry applications due to their structural efficiency as stiffened panels, shells and tanks. Composite grid-stiffened structures provide a better strength-to-weight ratio for these applications resulting in lower weight and increased payload capacity. Composite grid-stiffened structures also provide an alternative to honeycomb-core structures. Grid stiffened structures have no moisture entrapment problems and require no core forming, placement and/or potting.
Current methods to fabricate composite grid-stiffened structures often have resin layers between each grid stiffener and the attached skin. Current fabrication methods utilize filament winding or fiber placement to lay down tow for both the stiffeners and skin, the advantage being that a single, integrated co-cured structure can be fabricated using automated machinery. However, this results in a laminate skin that is joined to the stiffeners with only the layer of resin forming the bond line and with no fibers crossing the bond-line.
Grid stiffeners commonly have unidirectional fiber placement. The resulting stiffener is strongest in the directions parallel to the stiffener axis and weakest in the directions transverse to its axis, because all the fibers are generally positioned parallel to the stiffener axis. Transverse stiffener strength is therefore a structural limitation. Further, if disbonding occurs anywhere in the stiffener/skin bond line, the disbanding can propagate throughout the structure unimpeded. The bond between the stiffeners and the body skin is not maximized because of the lack of interweaving between the skin and the parallel positioned stiffener fibers. Current composite grid-stiffened structures having only a layer of resin between the stiffeners and skin are therefore best suited for expendable structures, such as rocket launchers, and not for structures which require long life and damage resistance such as aircraft and similar vehicles.